Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Split sdhci driver in two parts: sdhci and sdhci_pci.
sdchi encapsulates a generic SD Host Controller logic that relies on
actual hardware driver for register access.

sdhci_pci implements driver for PCI SDHC controllers using new SDHCI
interface

No kernel config modifications are required, but if you load sdhc
as a module you must switch to sdhci_pci instead.

Import sdhci (PCI SD Host Controller) driver.
Driver supports PCI devices with class 8 and subclass 5 according to
SD Host Controller Specification.

Update NOTES, enable module and static build.
Enable related mmc and mmcsd modules build.

Discussed on: mobile@, current@